RBS1223 was discovered as a bright X-ray source in the ROSAT All-Sky Survey.
Even when imaged with the giant 10m Keck telescope on Hawaii the
X-ray source remained without optical counterpart, suggesting an
isolated neutron star as source of the X-ray radiation. The upper left
figure shows X-ray contours overlaid over the Keck optical image).
These bizarre objects have about 1 solar mass concentrated in a radius
of only about 10 kilometer. The surface temperature of 1 Million degree
makes this star bright at X-ray wavelengths despite its negligible size
(for comparison, our sun has a radius of 700000 kilometers).
RBS1223 was re-observed with the Chandra X-ray observatory on June 24, 2000,
for a total of 9000 seconds. The Chandra X-ray image is shown in the
upper right figure. Due to the larger collecting area compared with ROSAT
much more X-ray photons were collected per unit time interval, thus
allowing the study of the time variable X-ray emission.
A 5.157 second
periodicity was found, the phase-averaged X-ray light curve is shown in
the lower figure.
The period found in the recent Chandra-observations is significantly longer
than that found with ROSAT in the past.
Some breaking mechanism is acting making the neutron star"s period spinning
down.
The most likely physical mechanism is the decay of a superstrong magnetic
field of the order of 100 TeraGauss (10^14 Gauss), i.e. more than 10^14 times
stronger than the mean terrestrial magnetic field.
Only a handful of these strange stars are known today.
They are under intensive investigation with the NASA Chandra and the ESA
XMM-Newton X-ray observatories.
( credit: Hambaryan, Hasinger, Schwope, with Schulz (MIT) )
